I had just fixed a network driver bug when I upgrade the kernel from 2.6.23 to 2.6.27 for our MIPS platform. It takes couple of weeks. The original problem appears when the NAPI was used in network driver and I changed the net_poll function accordingly. Then, I got kernel panic with memory access failure. After long time debugging, I found that there is some problem when the driver tries to figure out the address of skb out of the skb->data structure. This is weird because the same code was used in both 2.6.23 kernel and 2.6.27 kernel. The original author of the network driver gave some hints that he had experienced similar problem when he was creating a network driver for our next generation chip, based on the old driver. He mentioned that the original driver was confused about the physical/virtual address when accessing the dma'ed memory. This is quite helpful. I spend a whole day dig into this issue, and studied the new driver for next generation chip. After replacing the memory allocation function for the skb buffer, I finally got the proper method to access the memory. I've learned about the physical/virtual/bus address when accessing memory in kernel. The principle was simple as stated by Linus, "use virtual address when accessing memory in kernel, and use bus address when the memory was given to device". In some architecture, the bus address is identical to physical address. Never use physical address directly. The functions: phys_to_virt, virt_to_bus, bus_to_virt, virt_to_phys, are all helper functions.
It seems that we still had a lot of bugs in our network driver. Apparently, our engineers haven't had enough knowledge creating drivers in Linux. Most of their experience was in VxWorks, with flat memory model.
